Heat pump water heater and method

ABSTRACT

A heat pump water heater and method is disclosed. The heat pump water heater includes a stratified water tank configured to hold water of different temperatures therein and a heat exchange unit. The heat exchange unit includes a pump configured to remove water from a bottom of the stratified tank, a heat pump having a condenser configured to receive the water from the pump and supply heat to the water flowing through the condenser such that the condenser heats the water to a pre-determined temperature, and a resistance heater configured to receive the heated water from the condenser. The resistance heater supplies additional heat to the water if the pre-determined temperature of the water exiting the condenser does not equal or exceed a pre-determined set-point. The heated water from the resistance heater is supplied to a top of the stratified tank for use by a residence or utility.

BACKGROUND OF THE INVENTION

This application relates to a heat pump water heater and method and,more particularly, to a heat pump water heater adapted to increaseefficiency and, thereby control electric load.

In pursuit of energy efficiency, new technologies are sought to providecost effective energy savings compared to legacy technologies. One suchtechnology is Heat Pump Water Heaters (HPWHs). A residential electricwater heater draws approximately 4.5 kW (15 kBTU/hr) when running, andfor a family of four, consumes nearly 4.8 MWh (16.4 MMBTU) annually.Water heating results in nearly 11% of the residential carbon dioxidebudget and over 11% of the typical utility bill.

In 2015, new federal regulations are expected to require that electricwater heaters larger than 55 gallons have energy factors close to 2.0which will essentially eliminate residential electric resistance waterheaters and, thereby push HPWHs to the forefront. The one exceptionbeing considered is for electric resistance water heaters that provide“utility services”. The exception is being considered in part due to theinability of heat pumps to provide the same services.

Accordingly, there are problems to address as a result of the pursuit ofenergy efficiency and the inability of heat pumps to provide certainservices. The first is the ability for water heaters to provide gridservices and the second is to provide those services in an energyefficient manner. Current electric resistance water heaters are capableof providing grid services, such as providing demand response, frequencyregulation, and other services. However, electric resistance elementsare highly inefficient compared with heat pumps. Heat pump water heatersas they currently are made can provide less of the same services, orprovide those services only when the heat pump is not running, therebyeliminating the efficiency gains. Also, today's heat pumps are not asefficient as they could be.

BRIEF SUMMARY OF THE INVENTION

These and other shortcomings of the prior art are addressed by thepresent invention, which provides a heat pump water heater adapted toincrease efficiency and provide grid services such as Demand Response.

According to one aspect of the invention, a heat pump water heaterconfigured to provide grid related services such as Demand Responseincludes a stratified water tank configured to hold water of differenttemperatures therein and a heat exchange unit. The heat exchange unitincludes a pump configured to remove water from a bottom of thestratified tank, a heat pump having a condenser configured to receivethe water from the pump and supply heat to the water flowing through thecondenser such that the condenser heats the water to a pre-determinedtemperature, and a resistance heater configured to receive the heatedwater from the condenser. The resistance heater supplies additional heatto the water if the pre-determined temperature of the water exiting thecondenser does not equal or exceed a pre-determined set-point. Theheated water from the resistance heater is supplied to a top of thestratified tank for use by a residence or utility.

According to another aspect of the invention, a method of providing hotwater to a residence or utility includes the steps of providing a heatpump water heater, having a stratified water tank, and a heat exchangeunit having a heat pump and an in-line resistance heater. The methodfurther includes the steps of pulling water from a bottom of thestratified water tank and supplying it to the heat pump, using the heatpump to heat the water to a higher temperature, using the in-lineresistance heater to provide additional heat to the water exiting theheat pump, and storing the heated water in a top of the stratified watertank for use by a residence or utility.

BRIEF DESCRIPTION OF THE INVENTION

The subject matter that is regarded as the invention may be bestunderstood by reference to the following description taken inconjunction with the accompanying drawing figures in which:

FIG. 1 shows a prior art heat pump water heater;

FIG. 2 shows a prior art heat pump water heater with externalcondensers; and

FIG. 3 shows a heat pump water heater according to an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, a typical prior art heat pump water heater(HPWH) is shown in FIG. 1. The key elements of a typical HPWH are (1) aheat pump with a condensing coil that is integral to the tank (typicallywrapped around the outside of the tank, occasionally immersed as shown)and (2) one or two electric resistance elements used for backup heatwhich are immersed in the tank and separate from all heat pumpcomponents.

Manufacturers have also developed devices with external condensers, asshown in FIG. 2. An advantage of such a configuration is that thecondensing coil always has low-temperature water entering because ofstratification, which is good for efficiency. A wrap-around or immersedcondenser, shown in FIG. 1, heats the tank in bulk, and efficiencydecreases as the average tank temperature increases.

In general, heat pump water heaters use electricity to move heat fromone place to another instead of generating heat directly. Therefore,they can be two to three times more energy efficient than conventionalelectric resistance water heaters. Instead of heating stored waterdirectly with a conventional electric element or with a burner, as inthe case of a gas unit, a heat pump water heater transfers availableheat from the ambient air, intensifies the heat and transfers the heatinto the water, a far more cost and energy-efficient process.

Referring now to FIG. 3, a heat pump water heater in accordance with anembodiment of the invention is illustrated and shown generally atreference numeral 10. The heater 10 includes a stratified water tank 11configured to hold both cold water coming in from a utility and hotwater supplied by a heat exchange unit 12. The heat exchange unit 12includes a heat pump 13, a resistance heater 14, and a pump 16. The heatpump 13 includes an expansion valve 17, evaporator 18, fan 19,compressor 20, and condenser 21.

As shown, water from a utility is supplied to the tank 11. Pump 16 drawsthe water from the bottom of the tank 11 so that the water can be heatedby the condenser 21. The pump 16 may be a variable speed pump or mayconsist of a non-variable speed pump with a regulating valve to vary theflow-rate of the water. The control for the flow-rate is based on theoutlet water temperature.

In order for condenser 21 to provide heat to the water flowingtherethrough, a refrigerant flowing through the heat pump 13 issubjected to a vapor-compression cycle. More particularly, starting withthe evaporator 18, a low-pressure refrigerant enters the evaporator 18where the refrigerant absorbs heat from its surroundings and boils intoa gaseous state. The refrigerant is then compressed by the compressor 20into a hot and highly pressurized vapor. The refrigerant is then cooledby the condenser 21 by transferring heat into the cold water from thetank 11 flowing through the condenser 21. In transferring heat from therefrigerant to the water, the refrigerant becomes a high-pressure,moderate temperature liquid. The refrigerant then flows through theexpansion valve 17 where the refrigerant becomes a low-pressure liquid.The cycle then starts again.

The water flowing through the condenser 21 is heated to a set-point orlower. The water exits the condenser 21 and flows into an in-lineresistance heater 14. The in-line resistance heater 14 makes sure thatthe water is heated to the set-point. In other words, if the condenser21 heats the water to a temperature below the set-point, then thein-line resistance heater 14 provides the additional heat to bring thetemperature of the water to the set-point.

In addition to providing additional heat to bring the water up to theset-point, the resistance heater 14 may be used to provide:

-   -   Higher-power operation as a “Demand Response: Load Up” signal        response.    -   Ancillary services by modulating power to the element via pulse        width modulation or other control.    -   Potential elevated-temperature storage in advance of “Demand        Response: Load Down” events or for load shifting, with the heat        pump providing heat as high as is efficient and the resistance        element providing additional temperature boost.    -   Additional heating capacity as needed, either for fast recovery        or to boost capacity due to low ambient temperatures.

The compressor 20 may be of variable or fixed capacity. Depending onwhether the compressor 20 is of variable or fixed capacity there will betwo or three control points: (1) the water flow rate, (2) the electricelement heat capacity (or lack thereof), and (3) the heat pump heatcapacity (or lack thereof). These three variables are controlled inconcert to provide the desired heating output and power response. Thesize of the electric resistance element is a design variable and theremay be advantages to smaller or larger elements for different types ofutility usage.

There are several advantages to a heat pump water heater utilizing anin-line resistance heater, including:

-   -   Incorporating a heat pump into grid-tied devices, rather than        electric resistance alone, is far more efficient and will result        in lower operating costs.    -   Having an electric resistance element incorporated into a heat        pump water heater allows fast-response frequency modulation and        other benefits which may not be feasible with a heat pump-only        system.    -   External condenser heat pump water heaters with a high degree of        tank stratification have a higher annual efficiency than        wrap-around condensers.    -   Operating the heat pump and electric resistance element together        allows the heat pump to heat to a lower temperature, which is        good for heat pump efficiency.    -   Tank stratification can allow some cool water to be kept at the        bottom of the tank “on reserve” to be heated during “Demand        Response” events. This cannot be done with an unstratified tank,        where the only option would be to heat above set-point        (inefficient for heat pumps, requires additional safety        hardware).    -   Using wrap-around condensers and tank-immersed electric        resistance elements to provide a similar service generally means        turning off the heat pump when the elements are active, to keep        current down.

The foregoing has described a heat pump water heater and method forcontrolling electric load. While specific embodiments of the presentinvention have been described, it will be apparent to those skilled inthe art that various modifications thereto can be made without departingfrom the spirit and scope of the invention. Accordingly, the foregoingdescription of the preferred embodiment of the invention and the bestmode for practicing the invention are provided for the purpose ofillustration only and not for the purpose of limitation.

We claim:
 1. A heat pump water heater configured to provide grid relatedservices such as Demand Response, comprising: (a) a stratified watertank configured to hold water of different temperatures therein; and (b)a heat exchange unit, comprising: (i) a pump configured to remove waterfrom a bottom of the stratified tank; (ii) a heat pump having acondenser configured to receive the water from the pump and supply heatto the water flowing through the condenser such that the condenser heatsthe water to a pre-determined temperature; and (iii) a resistance heaterconfigured to receive the heated water from the condenser and supplyadditional heat to the water if the pre-determined temperature of thewater exiting the condenser does not equal or exceed a pre-determinedset-point, wherein the heated water from the resistance heater issupplied to a top of the stratified tank for use by a residence orutility.
 2. The heat pump water heater according to claim 1, wherein theheat pump further includes: (a) an evaporator configured to provide heatto a low-pressure, low-temperature refrigerant flowing therethrough; (b)a compressor configured to receive the refrigerant from the evaporatorand compress the refrigerant into a high-temperature, high-pressurerefrigerant, wherein the high-temperature, high-pressure refrigerant issupplied to the condenser for heating of the water flowing therethrough;and (c) an expansion valve configured to receive the refrigerant fromthe condenser in a high-pressure, moderate temperature state and expandit into a low-pressure, low-temperature state for use in the evaporator.3. The heat pump water heater according to claim 2, wherein thecompressor is of a variable capacity design.
 4. The heat pump waterheater according to claim 1, wherein the resistance heater is configuredto provide elevated temperature storage in advance of Demand Responseevents or for load shifting.
 5. The heat pump water heater according toclaim 1, wherein the resistance heater is configured to provideadditional heating capacity as needed for fast recovery and to boostcapacity due to low ambient temperatures.
 6. The heat pump water heateraccording to claim 1, wherein the resistance heater provides theadditional heat to the water so that the heat pump can operate atmaximum efficiency.
 7. A method of providing hot water to a residence orutility, comprising the steps of: (a) providing a heat pump waterheater, having: (i) a stratified water tank; and (ii) a heat exchangeunit having a heat pump and an in-line resistance heater; (b) pullingwater from a bottom of the stratified water tank and supplying it to theheat pump; (c) using the heat pump to heat the water to a highertemperature; (d) using the in-line resistance heater to provideadditional heat to the water exiting the heat pump; and (e) storing theheated water in a top of the stratified water tank for use by aresidence or utility.